1. Technical Field of the Invention
The present invention relates generally to spark plugs for internal combustion engines. More particularly, the invention relates to a spark plug which has a center electrode, a first ground electrode aligned with the center electrode in an axial direction of the spark plug to form a first spark gap across which sparks are discharged in normal conditions, and a second ground electrode aligned with the center electrode in a radial direction of the spark plug to form a second spark gap across which sparks are discharged when an insulator of the spark plug is fouled with carbon deposits.
2. Description of the Related Art
Existing spark plugs with a plurality of ground electrodes generally include a tubular metal shell, a cylindrical insulator, a center electrode, a first ground electrode, and a second ground electrode.
The metal shell has a threaded portion for fitting the spark plug into a combustion chamber of an engine.
The insulator has a bore formed therethrough in an axial direction thereof and is fixed in the metal shell such that an end thereof protrudes from an end of the metal shell.
The center electrode is secured in the bore of the insulator and includes a first diameter portion, a second diameter portion, and an intermediate portion provided between the first and second diameter portions. The first diameter portion is positioned outside the bore of the insulator and has a free end. The second diameter portion has a diameter greater than that of the first diameter portion. The intermediate portion has a first interface with the first diameter portion and a second interface with the second diameter portion and tapers from the second interface to the first interface.
The first ground electrode has a base end joined to the end of the metal shell and a tip portion that faces the end of the first diameter portion of the center electrode through a first spark gap in the axial direction of the insulator.
The second electrode has a base end joined to the end of the metal shell and a tip portion that faces the second interface of the intermediate portion of the center electrode with the second diameter portion of the same through a second spark gap in a radial direction of the insulator.
In such a spark plug, normal sparks are discharged across the first spark gap in normal conditions of the spark plug.
However, when the insulator of the spark plug is fouled with carbon deposits, “side sparks” may be discharged, instead of the normal sparks, across the second spark gap.
Specifically, with reference to FIG. 9, the electrical field formed in the spark plug tends to concentrate on the second interface 3bc of the intermediate portion 3b of the center electrode 3 with the second diameter portion 3c of the same. In other words, the electrical field generally has a large strength on the interface 3bc. As a result, when the insulator 2 is fouled with carbon deposits, side sparks may be discharged which jump from the interface 3bc to an end 5a of the tip portion 5t of the second ground electrode 5, thereby burning off the carbon deposits.
When the insulator 2 is fouled with carbon deposits, the insulation properties of the insulator 2 are degraded, or even lost. However, with the above arrangement, the spark plug can clean the insulator 2 by itself through burning off the carbon deposits with the side sparks, thereby recovering the insulation properties of the insulator 2.
Japanese Unexamined Patent Publication No. 2001-93645 and Japanese Patent No. 3272615 disclose spark plugs designed to maximize such a “self-clean” effect by burning off the carbon deposits with side sparks. In those spark plugs, the second interface 3bc of the intermediate portion 3b with the second diameter portion 3c is positioned inside the bore 2b of the insulator 2, and side sparks are discharged along a discharge path Z as shown in FIG. 9, which includes the whole length between the inner and outer edge of the end 2a of the insulator 2.
However, in the above spark plugs, since side sparks pass through the whole length between the inner and outer edge of the end 2a of the insulator 2, a channeling problem tends to occur. The channeling problem here denotes a phenomenon in which the end 2a of the insulator 2 is melted due to the heat energy transferred thereto from the side sparks that creep through the end 2a, thus forming channels along the discharge path Z on the end 2a. 
On the other hand, Japanese Patent No. 3140006 discloses a spark plug in which the second interface 3bc of the intermediate portion 3b with the second diameter portion 3c is positioned outside the bore 2b of the insulator 2. However, in the spark plug, the discharge path Z, along which side sparks are discharged, still includes the whole length between the inner and outer edge of the end 2a of the insulator 2. Consequently, the channeling problem still tends to occur.